For example, an electric car, a hybrid vehicle, or the like has a high-voltage battery as a power source for driving a motor. Such a high-voltage battery connects a plurality of unit cells of a secondary battery (rechargeable battery) such as a nickel-hydrogen battery and a lithium battery in series to provide a high voltage.
In the secondary battery, each unit cell is charged with the same power and is discharged at the same power. Accordingly, if the individual unit cells differently deteriorate, the secondary battery will easily get an overcharge or overdischarge state. To prevent the overcharge or overdischarge state of the secondary battery, the charging state of each unit cell must be checked. Previously, a plurality of (for example, 55) unit cells are divided into, for example, five blocks (namely, 11 unit cells per block) and a cell voltage of each unit cell contained in each block is measured in real time with a voltage detecting IC provided for each block, to monitor if the voltage is abnormal.
As to the voltage monitoring, the voltage detecting IC measures voltages of the unit cells (for example, 11 pieces) in the block, converts a detected analog voltage signal into a digital signal with an A/D converter contained in the voltage detecting IC, and transmits the digital signal to a main microcomputer.
Each voltage detecting IC may involve an error in the accuracy of unit cell voltage measurement. The error, if exists, must be detected and calibrated. There are known voltage measurement accuracy calibration methods, such as one disclosed in Japanese Unexamined Patent Application Publication No. 2005-62028. This related art computes an average of unit cell voltages of each block (each voltage detecting IC) and sets the average as a representative value of the block. According to the representative value, the related art detects a detection error in each unit cell voltage, and if the error is large, corrects the error.